Tire comprising a composition essentially free of guanidine derivative and comprising a hydroxyalkylpiperazine

ABSTRACT

A tire comprises a rubber composition based on at least one diene elastomer, a reinforcing filler predominantly comprising silica, a crosslinking system, and a hydroxyalkylpiperazine of formula (I): 
     
       
         
         
             
             
         
       
     
     in which R 1  represents a linear or branched alkyl group comprising from 1 to 18 carbon atoms, R 2  represents a linear or branched alkylene group comprising from 1 to 18 carbon atoms, it being understood that R 1  and R 2  can bear one or more additional hydroxyl groups. The rubber composition is essentially devoid of guanidine derivative.

The invention relates to tyres and more particularly to those for whichthe composition comprises a hydroxyalkylpiperazine.

Since fuel savings and the need to protect the environment have become apriority, it has proved necessary to produce tyres having a reducedrolling resistance, without adversely affecting other properties of thetyre. Manufacturers have developed tyre compositions which make itpossible to reduce this rolling resistance, in particular by theintroduction of silica into the mixtures as reinforcing filler.

Nevertheless, manufacturers are always looking for solutions for furtherlowering the rolling resistance of tyres and it is in this context thatthe Applicant Companies have discovered, surprisingly, that theintroduction of a hydroxyalkylpiperazine, as replacement for guanidinederivatives, makes it possible to reduce the hysteresis of diene rubbercompositions comprising silica as predominant reinforcing filler.

Furthermore, this solution exhibits numerous other advantages incomparison with the compositions of the prior art and in particular theuse of a compound which is cheaper and which makes possible animprovement in the environmental footprint.

Within the meaning of the invention, the hydroxyalkylpiperazines arepiperazines substituted on their nitrogen atoms with alkyl groups and atleast one of the carbon-based chains of which bears at least onehydroxyl function.

These hydroxyalkylpiperazines have never been used as proposed by theApplicant Companies to replace guanidine derivatives in diene elastomermixtures comprising silica as predominant filler, with the advantagespresented above.

The invention thus relates to a tyre comprising a rubber compositionbased on at least one diene elastomer, a reinforcing fillerpredominantly comprising silica and a crosslinking system, the saidcomposition being essentially devoid of guanidine derivative andadditionally comprising a hydroxyalkylpiperazine of formula (I):

in which R₁ represents a linear or branched alkyl group comprising from1 to 18 carbon atoms, R₂ represents a linear or branched alkylene groupcomprising from 1 to 18 carbon atoms, it being understood that R₁ and R₂can bear one or more additional hydroxyl groups.

Preferably, the invention relates to a tyre as defined above, in whichthe composition comprising the hydroxyalkylpiperazine comprises lessthan 0.45 phr of guanidine derivative and preferably less than 0.4 phr.

Preferably again, the invention relates to a tyre as defined above, inwhich the content of hydroxyalkylpiperazine is from 0.1 to 8 phr,preferably from more than 0.15 phr to 7 phr. More preferably, thecontent of hydroxyalkylpiperazine is from 0.2 to 5 phr, preferably frommore than 0.25 phr to 4 phr.

More preferably, the invention relates to a tyre as defined above, inwhich R₁ and R₂ comprise from 2 to 12 carbon atoms, more preferably from2 to 6 carbon atoms and very preferably from 2 to 4 carbon atoms.Preferably again, R₁ and R₂ are linear carbon-based chains. Preferablyagain, the hydroxyalkylpiperazine of formula (I) bears an additionalhydroxyl group on R₁ or R₂, preferably on R₁.

Preferably, the invention relates to a tyre as defined above, in whichthe silica content is from 30 to 150 phr.

Preferably again, the invention relates to a tyre as defined above, inwhich the reinforcing filler comprises carbon black, in a minor amount.Preferably, the content of carbon black is from 0.5 to 50 phr.

More preferably, the invention relates to a tyre as defined above, inwhich the diene elastomer is selected from the group consisting ofpolybutadienes, synthetic polyisoprenes, natural rubber, butadienecopolymers, isoprene copolymers (such as butadiene/styrene copolymers,isoprene/butadiene copolymers, isoprene/styrene copolymers andisoprene/butadiene/styrene copolymers) and the mixtures of theseelastomers.

According to a preferred form, the invention relates to a tyre asdefined above, in which the diene elastomer is predominantly composed ofnon-isoprene diene elastomer.

Preferably, the invention relates to a tyre as defined above, in whichthe diene elastomer is composed of 100 phr of non-isoprene dieneelastomer.

Preferably again, the invention relates to a tyre as defined above, inwhich the non-isoprene diene elastomer is selected from the groupconsisting of polybutadienes, butadiene copolymers, isoprene copolymers(such as butadiene/styrene copolymers, isoprene/butadiene copolymers,isoprene/styrene copolymers and isoprene/butadiene/styrene copolymers)and the mixtures of these elastomers.

More preferably, the invention relates to a tyre as defined above, inwhich the composition additionally comprises a plasticizer preferablychosen from plasticizing resins, extending oils and their mixtures.

Preferably, the invention relates to a tyre as defined above, in whichthe plasticizer content is from 5 to 100 phr.

Preferably, the invention relates to a tyre as defined above, in whichthe composition is in addition devoid of zinc or comprises less than 0.5phr, preferably less than 0.3 phr, thereof.

Preferably again, the invention relates to a tyre as defined above,additionally comprising a coupling agent.

Preferably, the invention relates to a tyre as defined above, in whichthe coupling agent is a hydroxysilane polysulphide corresponding to thegeneral formula (III):

(HO)_(a)R_((3-a))Si—R′→S_(x)—R′—SiR_((3-b))(OH)_(b)  (III)

in which:

-   -   the R radicals, which are identical or different, are        hydrocarbon groups preferably comprising from 1 to 15 carbon        atoms;    -   the R′ radicals, which are identical or different, are divalent        connecting groups preferably comprising from 1 to 18 carbon        atoms;    -   a and b, which are identical or different, are equal to 1 or 2;    -   x is a number greater than or equal to 2.

More preferably, the invention relates to a tyre as defined above, inwhich the coupling agent is a monohydroxysilane in which a and b areequal to 1.

More preferably still, the invention relates to a tyre as defined above,in which the R radicals are chosen from linear or branched C₁-C₆ alkyls,C₅-C₈ cycloalkyls or a phenyl radical; the R′ radicals are chosen fromC₁-C₁₈ alkylenes or C₆-C₁₂ arylenes. More particularly, the R radicalsare chosen from C₁-C₆ alkyls and the R′ radicals from C₁-C₁₀ alkylenes.

Very preferably, the invention relates to a tyre as defined above, inwhich the hydroxysilane is a monohydroxysilane polysulphide of formula(IV):

in which the R radicals are C₁-C₃ alkyls, preferably methyl; the R′radicals are C₁-C₄ alkylenes, preferably methylene, ethylene orpropylene; x is greater than or equal to 2. More particularly, thehydroxysilane is a bis(propyldimethylsilanol) polysulphide of specificformula (IVa):

Preferably, the invention relates to a tyre as defined above, in whichthe composition comprising the hydroxyalkylpiperazine is the compositionof a tyre layer selected from the group consisting of all or part of thetread, of all or part of the tyre belt and their combinations.

Preferably, the invention relates to a tyre as defined above, in whichthe composition comprising the hydroxyalkylpiperazine is that of all orpart of the tread.

The tyre according to the invention can comprise the compositiondescribed above in one or more of its various layers, such as, forexample, the tread, the plies of the tyre belt, the carcass ply or anyother layer. Within the meaning of the present patent application, thetyre tread denotes the rubber layer in contact with the ground,completely (that is to say, over its entire thickness) or a portion ofthe latter (including the underlayer), in particular when it is composedof several layers.

Preferably, the invention relates to a tyre as defined above, in whichthe composition comprising the hydroxyalkylpiperazine is the compositionof a tyre layer selected from the group consisting of all or part of thetread, of all or part of the tyre belt and their combinations.

Preferably, the invention relates to a tyre as defined above, in whichthe composition specified is that of all or part of the tread.

Preferably, the tyre according to the invention will be chosen from thetyres intended to equip a two-wheel vehicle, a passenger vehicle, oralso a heavy-duty vehicle (that is to say, underground, bus, off-roadvehicles, heavy road transport vehicles, such as lorries, tractors ortrailers), or also aircraft, construction equipment, heavy agriculturalvehicles or handling vehicles.

I—CONSTITUENTS OF THE COMPOSITION

The rubber compositions according to the invention are based on thefollowing constituents: at least one diene elastomer, a reinforcingfiller, a crosslinking system and a hydroxyalkylpiperazine.

The expression “composition based on” should be understood as meaning acomposition comprising the mixture and/or the product of the in situreaction of the various base constituents used, some of theseconstituents being able to react and/or being intended to react with oneanother, at least partially, during the various phases of manufacture ofthe composition or during the subsequent curing, modifying thecomposition as it is prepared at the start. Thus, the compositions asemployed for the invention can be different in the non-crosslinked stateand in the crosslinked state.

In the present description, unless expressly indicated otherwise, allthe percentages (%) shown are percentages by weight. Furthermore, anyinterval of values denoted by the expression “between a and b”represents the range of values extending from more than a to less than b(that is to say, limits a and b excluded), whereas any interval ofvalues denoted by the expression “from a to b” means the range of valuesextending from a up to b (that is to say, including the strict limits aand b).

I-1 Diene Elastomer

The compositions can comprise just one diene elastomer or a mixture ofseveral diene elastomers.

It should be remembered here that elastomer (or “rubber”, the two termsbeing regarded as synonymous) of the “diene” type should be understood,in a known way, to mean an (one or more is understood) elastomerresulting at least in part (i.e., a homopolymer or a copolymer) fromdiene monomers (monomers bearing two conjugated or non-conjugatedcarbon-carbon double bonds).

The diene elastomers can be classified into two categories: “essentiallyunsaturated” or “essentially saturated”. Generally, “essentiallyunsaturated” is understood to mean a diene elastomer resulting at leastin part from conjugated diene monomers having a content of units ofdiene origin (conjugated dienes) which is greater than 15% (mol %); thusit is that diene elastomers, such as butyl rubbers or copolymers ofdienes and α-olefins of EPDM type, do not come within the precedingdefinition and can in particular be described as “essentially saturated”diene elastomers (low or very low content, always less than 15%, ofunits of diene origin). In the category of “essentially unsaturated”diene elastomers, “highly unsaturated” diene elastomer is understood tomean in particular a diene elastomer having a content of units of dieneorigin (conjugated dienes) which is greater than 50%.

Given these definitions, diene elastomer capable of being used in thecompositions according to the invention is understood more particularlyto mean:

(a)—any homopolymer obtained by polymerization of a conjugated dienemonomer having from 4 to 12 carbon atoms;(b)—any copolymer obtained by copolymerization of one or more conjugateddienes with one another or with one or more vinylaromatic compoundshaving from 8 to 20 carbon atoms;(c)—a ternary copolymer obtained by copolymerization of ethylene and ofan α-olefin having from 3 to 6 carbon atoms with a non-conjugated dienemonomer having from 6 to 12 carbon atoms, such as, for example, theelastomers obtained from ethylene and propylene with a non-conjugateddiene monomer of the abovementioned type, such as, in particular,1,4-hexadiene, ethylidenenorbornene or dicyclopentadiene;(d)—a copolymer of isobutene and of isoprene (butyl rubber) and also thehalogenated versions, in particular chlorinated or brominated versions,of this type of copolymer.

Although it applies to any type of diene elastomer, a person skilled inthe art of tyres will understand that the present invention ispreferably employed with essentially unsaturated diene elastomers, inparticular of the above type (a) or (b).

The following are suitable in particular as conjugated dienes:1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁-C₅alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or2-methyl-3-isopropyl-1,3-butadiene, aryl-1,3-butadiene, 1,3-pentadieneor 2,4-hexadiene. The following, for example, are suitable asvinylaromatic compounds: styrene, ortho-, meta- or para-methylstyrene,the “vinyltoluene” commercial mixture, para-(tert-butyl)styrene,methoxystyrenes, chlorostyrenes, vinylmesitylene, divinylbenzene orvinylnaphthalene.

The copolymers can comprise between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinylaromatic units. The elastomerscan have any microstructure, which depends on the polymerizationconditions used, in particular on the presence or absence of a modifyingand/or randomizing agent and on the amounts of modifying and/orrandomizing agent employed. The elastomers can, for example, be block,statistical, sequential or microsequential elastomers and can beprepared in dispersion or in solution; they can be coupled and/orstar-branched or else functionalized with a coupling and/orstar-branching or functionalization agent. Mention may be made, forexample, for coupling to carbon black, of functional groups comprising aC—Sn bond or aminated functional groups, such as aminobenzophenone, forexample; mention may be made, for example, for coupling to a reinforcinginorganic filler, such as silica, of silanol or polysiloxane functionalgroups having a silanol end (such as described, for example, in FR 2 740778, U.S. Pat. No. 6,013,718 and WO 2008/141702), alkoxysilane groups(such as described, for example, in FR 2 765 882 or U.S. Pat. No.5,977,238), carboxyl groups (such as described, for example, in WO01/92402 or U.S. Pat. No. 6,815,473, WO 2004/096865 or US 2006/0089445)or else polyether groups (such as described, for example, in EP 1 127909, U.S. Pat. No. 6,503,973, WO 2009/000750 and WO 2009/000752).Mention may also be made, as other examples of functionalizedelastomers, of elastomers (such as SBR, BR, NR or IR) of the epoxidizedtype.

These functionalized elastomers can be used as a blend with one anotheror with non-functionalized elastomers. For example, it is possible touse a silanol- or polysiloxane-functionalized elastomer having a silanolend, as a mixture with an elastomer coupled and/or star-branched withtin (described in WO 11/042507), the latter representing a content of 5%to 50%, for example of 25% to 50%.

The following are suitable: polybutadienes, in particular those having acontent (mol %) of 1,2-units of between 4% and 80% or those having acontent (mol %) of cis-1,4-units of greater than 80%, polyisoprenes,butadiene/styrene copolymers and in particular those having a Tg (glasstransition temperature (Tg, measured according to ASTM D3418) of between0° C. and −70° C. and more particularly between −10° C. and −60° C., astyrene content of between 5% and 60% by weight and more particularlybetween 20% and 50%, a content (mol %) of 1,2-bonds of the butadienepart of between 4% and 75% and a content (mol %) of trans-1,4-bonds ofbetween 10% and 80%, butadiene/isoprene copolymers, in particular thosehaving an isoprene content of between 5% and 90% by weight and a Tg of−40° C. to −80° C., or isoprene/styrene copolymers, in particular thosehaving a styrene content of between 5% and 50% by weight and a Tg ofbetween −5° C. and −60° C. In the case of butadiene/styrene/isoprenecopolymers, those having a styrene content of between 5% and 50% byweight and more particularly of between 10% and 40%, an isoprene contentof between 15% and 60% by weight and more particularly between 20% and50%, a butadiene content of between 5% and 50% by weight and moreparticularly of between 20% and 40%, a content (mol %) of 1,2-units ofthe butadiene part of between 4% and 85%, a content (mol %) oftrans-1,4-units of the butadiene part of between 6% and 80%, a content(mol %) of 1,2-plus 3,4-units of the isoprene part of between 5% and 70%and a content (mol %) of trans-1,4-units of the isoprene part of between10% and 50%, and more generally any butadiene/styrene/isoprene copolymerhaving a Tg of between −20° C. and −70° C., are suitable in particular.

To summarize, the diene elastomer of the composition is preferablyselected from the group of highly unsaturated diene elastomersconsisting of polybutadienes (abbreviated to “BRs”), syntheticpolyisoprenes (IRs), natural rubber (NR), butadiene copolymers, isoprenecopolymers and the mixtures of these elastomers. Such copolymers aremore preferably selected from the group consisting of butadiene/styrenecopolymers (SBRs), isoprene/butadiene copolymers (BIRs),isoprene/styrene copolymers (SIRs), isoprene/butadiene/styrenecopolymers (SBIRs), butadiene/acrylonitrile copolymers (NBRs),butadiene/styrene/acrylonitrile copolymers (NSBRs) or a mixture of twoor more of these compounds.

According to a specific embodiment, the composition comprises from 50 to100 phr of an SBR elastomer, whether an SBR prepared in emulsion(“ESBR”) or an SBR prepared in solution (“SSBR”).

According to another specific embodiment, the diene elastomer is anSBR/BR blend (mixture).

According to other possible embodiments, the diene elastomer is anSBR/NR (or SBR/IR), BR/NR (or BR/IR) or also SBR/BR/NR (or SBR/BR/IR)blend.

In the case of an SBR (ESBR or SSBR) elastomer, use is made inparticular of an SBR having a moderate styrene content, for example ofbetween 20% and 35% by weight, or a high styrene content, for examplefrom 35% to 45%, a content of vinyl bonds of the butadiene part ofbetween 15% and 70%, a content (mol %) of trans-1,4-bonds of between 15%and 75% and a Tg of between −10° C. and −55° C.; such an SBR canadvantageously be used as a mixture with a BR preferably having morethan 90% (mol %) of cis-1,4-bonds.

According to another specific embodiment, the diene elastomer is apredominantly isoprene elastomer (that is to say, the fraction by weightof isoprene elastomer of which is the greatest, compared with thefraction by weight of the other elastomers). “Isoprene elastomer” isunderstood to mean, in a known way, an isoprene homopolymer orcopolymer, in other words a diene elastomer selected from the groupconsisting of natural rubber (NR), which may be plasticized or peptized,synthetic polyisoprenes (IRs), various isoprene copolymers and themixtures of these elastomers. Mention will in particular be made, amongisoprene copolymers, of isobutene/isoprene (butyl rubber—IIR),isoprene/styrene (SIR), isoprene/butadiene (BIR) orisoprene/butadiene/styrene (SBIR) copolymers. This isoprene elastomer ispreferably natural rubber or a synthetic cis-1,4-polyisoprene; use ispreferably made, among these synthetic polyisoprenes, of polyisopreneshaving a content (mol %) of cis-1,4-bonds of greater than 90%, morepreferably still of greater than 98%.

Preferably, according to another embodiment, the rubber compositionpredominantly comprises (that is to say, with the highest content byweight) a non-isoprene diene elastomer. “Non-isoprene diene elastomer”should be understood, within the meaning of the present patentapplication, as indicating an elastomer resulting at least in part(i.e., a homopolymer or a copolymer) from diene monomers (monomersbearing two carbon-carbon double bonds) other than isoprene. Thus,non-isoprene diene elastomers within the meaning of the presentdefinition thus also comprise copolymers comprising isoprene ascomonomer. Natural rubber and isoprene homopolymers (that is to say,composed of functionalized or non-functionalized isoprene monomers) areexcluded from the present definition. According to this preferredembodiment, all the abovementioned elastomers, with the exception ofnatural rubber and polyisoprenes, are suitable as non-isoprene dieneelastomer. In particular, it will be possible to use non-isoprene dieneelastomers preferably selected from the group of highly unsaturateddiene elastomers consisting of polybutadienes (abbreviated to “BRs”),butadiene copolymers, isoprene copolymers and the mixtures of theseelastomers. Such copolymers are more preferably selected from the groupconsisting of butadiene/styrene copolymers (SBRs), isoprene/butadienecopolymers (BIRs), isoprene/styrene copolymers (SIRs) andisoprene/butadiene/styrene copolymers (SBIRs). Still according to thispreferred embodiment, it will be understood that, in the case of a blendof elastomers, the total content of the “non-isoprene” elastomers mustbe greater than the total content of the elastomers selected from thegroup consisting of natural rubber, synthetic polyisoprenes and theirmixtures. Preferably, according to this embodiment, the content ofnon-isoprene diene elastomer is more than 50 phr, more preferably atleast 60 phr, more preferably at least 70 phr, more preferably still atleast 80 phr and very preferably at least 90 phr. In particular,according to this embodiment, the content of non-isoprene dieneelastomer is very preferably 100 phr.

According to another preferred embodiment of the invention, the rubbercomposition comprises a blend of a (one or more) “high Tg” dieneelastomer exhibiting a Tg of between −70° C. and 0° C. and of a (one ormore) “low Tg” diene elastomer of between −110° C. and −80° C., morepreferably between −105° C. and −90° C. The high Tg elastomer ispreferably selected from the group consisting of S-SBRs, E-SBRs, naturalrubber, synthetic polyisoprenes (exhibiting a content (mol %) ofcis-1,4-enchainments preferably of greater than 95%), BIRs, SIRs, SBIRsand the mixtures of these elastomers. The low Tg elastomer preferablycomprises butadiene units according to a content (mol %) at least equalto 70%; it preferably consists of a polybutadiene (BR) exhibiting acontent (mol %) of cis-1,4-enchainments of greater than 90%.

According to another specific embodiment of the invention, the rubbercomposition comprises, for example, between 30 and 90 phr, in particularbetween 40 and 90 phr, of a high Tg elastomer as a blend with a low Tgelastomer.

According to another specific embodiment of the invention, the dieneelastomer of the composition according to the invention comprises ablend of a BR (as low Tg elastomer) exhibiting a content (mol %) ofcis-1,4-enchainments of greater than 90% with one or more S-SBRs orE-SBRs (as high Tg elastomer(s)).

I-2 Reinforcing Filler

The tyre according to the invention comprises a composition whichpredominantly comprises silica as reinforcing filler. The term“predominant reinforcing filler” is understood to mean that whichexhibits the greatest content among the reinforcing fillers present inthe composition. In particular, the term “predominant reinforcingfiller” is understood to mean any reinforcing filler which represents atleast 50% by weight of the reinforcing fillers present, preferably morethan 50% and more preferably more than 60%.

The physical state under which the reinforcing filler is provided is notimportant, whether it is in the form of a powder, of microbeads, ofgranules, of beads or any other appropriate densified form.

The fraction by volume of reinforcing filler in the rubber compositionis defined as being the ratio of the volume of the reinforcing filler tothe volume of all the constituents of the composition, it beingunderstood that the volume of all the constituents is calculated byadding together the volumes of each of the constituents of thecomposition. The fraction by volume of reinforcing filler in acomposition is thus defined as the ratio of the volume of thereinforcing filler to the sum of the volumes of each of the constituentsof the composition; typically, this fraction by volume is between 10%and 30% and preferably between 15% and 25%. In an equivalent preferredway, the content of total reinforcing filler (carbon black and/orreinforcing inorganic filler, such as silica) is from 30 to 200 phr,more preferably from 30 to 150 phr and very preferably from 50 to 135phr.

According to a preferred embodiment of the invention, use is made of areinforcing filler comprising from 30 to 150 phr of silica, morepreferably from 50 to 130 phr of silica, and optionally carbon black;the carbon black, when it is present, is used in combination with thesilica, more preferably at a content of 0.5 to 50 phr, more preferablystill of 1 to 20 phr (in particular between 1 and 10 phr).

The composition can comprise one type of silica or a blend of severalsilicas. The silica used can be any reinforcing silica known to a personskilled in the art, in particular any precipitated or fumed silicaexhibiting a BET specific surface and a CTAB specific surface both ofless than 450 m²/g, preferably from 30 to 400 m²/g. Mention will bemade, as highly dispersible precipitated silicas (“HDSs”), for example,of the Ultrasil 7000 and Ultrasil 7005 silicas from Degussa, the Zeosil1165MP, 1135MP and 1115MP silicas from Rhodia, the Hi-Sil EZ150G silicafrom PPG, the Zeopol 8715, 8745 and 8755 silicas from Huber, treatedprecipitated silicas, such as, for example, the silicas “doped” withaluminium described in Application EP-A-0735088, or the silicas with ahigh specific surface as described in Application WO 03/16837.

The silica preferably has a BET specific surface of between 45 and 400m²/g, more preferably of between 60 and 300 m²/g.

These compositions can optionally also comprise, in addition to thecoupling agents, coupling activators, agents for covering the inorganicfillers or more generally processing aids capable, in a known way, byvirtue of an improvement in the dispersion of the filler in the rubbermatrix and of a lowering of the viscosity of the compositions, ofimproving their ability to be processed in the raw state, these agentsbeing, for example, hydrolysable silanes, such as alkylalkoxysilanes,polyols, fatty acids, polyethers, primary, secondary or tertiary amines,or hydroxylated or hydrolysable polyorganosiloxanes.

Use is made in particular of silane polysulphides, referred to as“symmetrical” or “unsymmetrical” depending on their specific structure,such as described, for example, in Applications WO 03/002648 (or US2005/016651) and WO 03/002649 (or US 2005/016650).

Suitable in particular, without the definition below being limiting, aresilane polysulphides referred to as “symmetrical”, corresponding to thefollowing general formula (II):

Z-A-S_(x)-A-Z,  (II)

in which:

-   -   x is an integer from 2 to 8 (preferably from 2 to 5);    -   A is a divalent hydrocarbon radical (preferably C₁-C₁₈ alkylene        groups or C₆-C₁₂ arylene groups, more particularly C₁-C₁₀, in        particular C₁-C₄, alkylenes, in particular propylene);    -   Z corresponds to one of the formulae below:

in which:

-   -   the R¹ radicals, which are substituted or unsubstituted and        identical to or different from one another, represent a C₁-C₁₈        alkyl, C₅-C₁₈ cycloalkyl or C₆-C₁₈ aryl group (preferably C₁-C₆        alkyl, cyclohexyl or phenyl groups, in particular C₁-C₄ alkyl        groups, more particularly methyl and/or ethyl);    -   the R² radicals, which are substituted or unsubstituted and        identical to or different from one another, represent a C₁-C₁₈        alkoxyl or C₅-C₁₈ cycloalkoxyl group (preferably a group chosen        from C₁-C₈ alkoxyls and C₅-C₈ cycloalkoxyls, more preferably        still a group chosen from C₁-C₄ alkoxyls, in particular methoxyl        and ethoxyl).

In the case of a mixture of alkoxysilane polysulphides corresponding tothe above formula (II), in particular normal commercially availablemixtures, the mean value of the “x” indices is a fractional numberpreferably of between 2 and 5, more preferably of approximately 4.However, the invention can also advantageously be carried out, forexample, with alkoxysilane disulphides (x=2).

Mention will more particularly be made, as examples of silanepolysulphides, of bis((C₁-C₄)alkoxyl(C₁-C₄)alkylsilyl(C₁-C₄)alkyl)polysulphides (in particular disulphides, trisulphides ortetrasulphides), such as, for example, bis(3-trimethoxysilylpropyl) orbis(3-triethoxysilylpropyl) polysulphides. Use is made in particular,among these compounds, of bis(3-triethoxysilylpropyl) tetrasulphide,abbreviated to TESPT, of formula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, orbis(triethoxysilylpropyl) disulphide, abbreviated to TESPD, of formula[(C₂H₅O)₃Si(CH₂)₃S]₂. Mention will also be made, as preferred examples,of bis(mono(C₁-C₄)alkoxyldi(C₁-C₄)alkylsilylpropyl) polysulphides (inparticular disulphides, trisulphides or tetrasulphides), moreparticularly bis(monoethoxydimethylsilylpropyl) tetrasulphide, such asdescribed in Patent Application WO 02/083782 (or US 2004/132880).

Mention will also be made, as coupling agent other than an alkoxysilanepolysulphide, of bifunctional POSs (polyorganosiloxanes) or else ofhydroxysilane polysulphides (R²=OH in the above formula II), such asdescribed in Patent Applications WO 02/30939 (or U.S. Pat. No.6,774,255) and WO 02/31041 (or US 2004/051210), or else of silanes orPOSs bearing azodicarbonyl functional groups, such as described, forexample, in Patent Applications WO 2006/125532, WO 2006/125533 and WO2006/125534.

In particular and preferably, the coupling agent can be a hydroxysilanepolysulphide (as described in the abovementioned documents)corresponding to the general formula (III):

(HO)_(a)R_((3-a))Si—R′—S_(x)—SiR_((3-b))(OH)_(b)  (III)

in which:

-   -   the R radicals, which are identical or different, are        hydrocarbon groups preferably comprising from 1 to 15 carbon        atoms;    -   the R′ radicals, which are identical or different, are divalent        connecting groups preferably comprising from 1 to 18 carbon        atoms;    -   a and b, which are identical or different, are equal to 1 or 2;    -   x is a number greater than or equal to 2.

The R radicals, which are identical or different, linear or branched andpreferably comprise from 1 to 15 carbon atoms, are more preferablychosen from alkyls, cycloalkyls or aryls, in particular from C₁-C₆alkyls, C₅-C₈ cycloalkyls and the phenyl radical. Mention will inparticular be made, among these radicals, by way of examples, of thoseselected from the group consisting of methyl, ethyl, n-propyl,isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, neopentyl, n-hexyl,2-ethylhexyl, n-octyl, isooctyl, cyclopentyl, cyclohexyl,2-methylcyclohexyl, phenyl, toluyl and benzyl.

More preferably still, the R radicals, which are identical or different,are C₁-C₃ alkyls (namely methyl, ethyl, n-propyl or isopropyl), veryparticularly chosen from methyl and ethyl.

The R′ radicals, which are identical or different and substituted orunsubstituted, are preferably saturated or unsaturated hydrocarbonradicals comprising from 1 to 18 carbon atoms, it being possible forthese R′ radicals to be interrupted within the hydrocarbon chain by atleast one heteroatom, such as O, S or N. Suitable in particular areC₁-C₁₈ alkylene groups or C₆-C₁₂ arylene groups, more particularlyC₁-C₁₀, in particular C₁-C₄, alkylenes, especially those chosen frommethylene, ethylene and propylene.

Preferably, in the hydroxysilane polysulphides corresponding to thegeneral formula (III), the hydroxysilane is a monohydroxysilane, that isto say that a and b are equal to 1. Preferably again, the R radicals arechosen from linear or branched C₁-C₆ alkyls, C₅-C₈ cycloalkyls or aphenyl radical; the R′ radicals are chosen from C₁-C₁₈ alkylenes orC₆-C₁₂ arylenes, and more particularly the R radicals are chosen fromC₁-C₆ alkyls and the R′ radicals are chosen from C₁-C₁₀ alkylenes.

Thus, very preferably, the hydroxysilane is a monohydroxysilanepolysulphide of formula (IV):

in which the R radicals are C₁-C₃ alkyls, preferably methyl; the R′radicals are C₁-C₄ alkylenes, preferably methylene, ethylene orpropylene; x is greater than or equal to 2. More particularly, thehydroxysilane can be a bis(propyldimethylsilanol) polysulphide ofspecific formula (IVa):

This product of formula (IVa) corresponds to the product D in theabovementioned document WO 02/31041 (or US 2004/051210).

In the rubber compositions in accordance with the invention, the contentof coupling agent is preferably between 2 and 15 phr, more preferablybetween 3 and 13 phr and more preferably still between 5 and 10 phr.

Use may be made, in addition to silica, of any type of reinforcingfiller known for its abilities to reinforce a rubber composition whichcan be used for the manufacture of tyres, for example an organic filler,such as carbon black, a reinforcing inorganic filler, such as alumina,or also a blend of these two types of filler.

All carbon blacks, in particular “tyre-grade” blacks, are suitable ascarbon blacks. Mention will more particularly be made, among the latter,of the reinforcing carbon blacks of the 100, 200 or 300 series (ASTMgrades), such as, for example, the N115, N134, N234, N326, N330, N339,N347 or N375 blacks, or else, depending on the applications targeted,the blacks of higher series (for example N660, N683 or N772). The carbonblacks might, for example, be already incorporated in an isopreneelastomer in the form of a masterbatch (see, for example, ApplicationsWO 97/36724 or WO 99/16600).

Mention may be made, as examples of organic fillers other than carbonblacks, of functionalized polyvinyl organic fillers, such as describedin Applications WO-A-2006/069792, WO-A-2006/069793, WO-A-2008/003434 andWO-A-2008/003435.

A person skilled in the art will understand that, as filler equivalentto silica described in the present section, use might be made of areinforcing filler of another nature, in particular organic nature,provided that this reinforcing filler is covered with a layer of silicaor else comprises functional sites, in particular hydroxyl sites, at itssurface which require the use of a coupling agent in order to form thebond between the filler and the elastomer.

I-3 Crosslinking System

The crosslinking system can be a vulcanization system; it is preferablybased on sulphur or sulphur donors and on primary vulcanizationaccelerator (preferably 0.5 to 10.0 phr of primary accelerator).Additional to this vulcanization system are optionally various knownsecondary vulcanization accelerators or vulcanization activators, suchas zinc oxide (preferably for 0.5 to 10.0 phr) or stearic acid or others(preferably for 0.5 to 5.0 phr each). The sulphur is used at a preferredcontent of between 0.5 and 10 phr, more preferably of between 0.5 and5.0 phr, for example between 0.5 and 3.0 phr when the invention isapplied to a tyre tread.

Use may be made, as (primary or secondary) accelerator, of any compoundcapable of acting as accelerator of the vulcanization of dieneelastomers in the presence of sulphur, in particular accelerators of thethiazole type and their derivatives and accelerators of the thiuram andzinc dithiocarbamate types. These accelerators are more preferablyselected from the group consisting of 2-mercaptobenzothiazyl disulphide(abbreviated to “MBTS”), N-cyclohexyl-2-benzothiazolesulphenamide(abbreviated to “CBS”), N,N-dicyclohexyl-2-benzothiazolesulphenamide(abbreviated to “DCBS”), N-(tert-butyl)-2-benzothiazolesulphenamide(abbreviated to “TBBS”), N-(tert-butyl)-2-benzothiazolesulphenimide(abbreviated to “TBSI”), zinc dibenzyldithiocarbamate (abbreviated to“ZBEC”) and the mixtures of these compounds. Preferably, use is made ofa primary accelerator of the sulphenamide type.

On the other hand, in the tyres according to the invention, thecomposition comprising the hydroxyalkylpiperazine necessary for therequirements of the invention is devoid of guanidine derivative orcomprises less than 0.5 phr thereof. Preferably, the composition iseither completely devoid of such compounds or it comprises less than0.45 phr, preferably less than 0.4 phr, more preferably less than 0.3phr, preferably less than 0.2 phr and very preferably less than 0.1 phrthereof. The term “guanidine derivative” is understood to mean theorganic compounds bearing a guanidine functional group as mainfunctional group, such as those known in tyre compositions, inparticular as vulcanization accelerators, for example diphenylguanidine(DPG) or di(ortho-tolyl)guanidine (DOTG).

According to a preferred form, in the tyre according to the invention,the composition comprising the hydroxyalkylpiperazine necessary for therequirements of the invention is also devoid of zinc or comprises lessthan 0.5 phr, preferably less than 0.3 phr, more preferably less than0.2 phr and very preferably less than 0.1 phr thereof.

I-4 Hydroxyalkylpiperazine

In order to advantageously replace the guanidine derivatives mentionedabove, the tyre according to the invention comprises ahydroxyalkylpiperazine of formula (I):

in which R₁ represents a linear or branched alkyl group comprising from1 to 18 carbon atoms, R₂ represents a linear or branched alkylene groupcomprising from 1 to 18 carbon atoms, it being understood that R₁ and R₂can be substituted with one or more additional hydroxyl groups.

Preferably, R₁ and R₂ comprise from 2 to 12 carbon atoms, morepreferably from 2 to 6 carbon atoms and very preferably from 2 to 4carbon atoms.

Preferably again, R₁ and R₂ are linear carbon-based chains.

Preferably, the hydroxyalkylpiperazine of formula (I) bears anadditional hydroxyl group on R₁ or R₂, preferably on R₁.

Preferably, the content of hydroxyalkylpiperazine in the composition iswithin a range varying from 0.1 to 8 phr, more preferably from 0.15 to 7phr, preferably from more than 0.2 to 5 phr and more preferably from0.25 to 4 phr.

By way of example, the hydroxyalkylpiperazine of formula (I) can be1,4-bis(2-hydroxyethyl)piperazine, commercially available in the powderform, for example from Sigma-Aldrich:

It may be noted that the hydroxyalkylpiperazine as defined below isintroduced in the free base form, that is to say not forming a salt withan inorganic or organic acid.

I-5 Other Possible Additives

The rubber compositions in accordance with the invention optionally alsocomprise all or a portion of the normal additives generally used inelastomer compositions intended in particular for the manufacture oftreads, such as, for example, pigments, protective agents, such asantiozone waxes, chemical antiozonants or antioxidants, plasticizingagents, such as those provided below, anti-fatigue agents, reinforcingresins, or methylene acceptors (for example novolak phenolic resin) ordonors (for example HMT or H3M).

According to a preferred embodiment, the composition according to theinvention additionally comprises a plasticizing agent. Preferably, thisplasticizing agent is a solid hydrocarbon resin (or plasticizing resin),an extending oil (or plasticizing oil) or a mixture of the two.

When it is included in the composition, the content of totalplasticizing agent is preferably greater than or equal to 5 phr, morepreferably from 5 to 100 phr, in particular from 10 to 80 phr, forexample from 15 to 70 phr.

According to a first preferred embodiment of the invention, theplasticizer is an extending oil which is liquid at 20° C., referred toas “low Tg”, that is to say which, by definition, exhibits a Tg of lessthan −20° C., preferably of less than −40° C.

Any extending oil, whether it is of aromatic or non-aromatic nature,known for its plasticizing properties with regard to diene elastomerscan be used. At ambient temperature (20° C.), these oils, which are moreor less viscous, are liquids (that is to say, as a reminder, substanceswhich have the ability to eventually assume the shape of theircontainer), in contrast in particular to plasticizing hydrocarbonresins, which are by nature solids at ambient temperature.

Extending oils selected from the group consisting of naphthenic oils(low or high viscosity, in particular hydrogenated or not), paraffinicoils, MES (Medium Extracted Solvates) oils, TDAE (Treated DistillateAromatic Extracts) oils, mineral oils, vegetable oils, etherplasticizers, ester plasticizers, phosphate plasticizers, sulphonateplasticizers and the mixtures of these compounds are particularlysuitable. For example, mention may be made of those which comprisebetween 12 and 30 carbon atoms, for example trioctyl phosphate. Mentionmay in particular be made, as examples of non-aqueous andwater-insoluble ester plasticizers, of the compounds selected from thegroup consisting of trimellitates, pyromellitates, phthalates,1,2-cyclohexanedicarboxylates, adipates, azelates, sebacates, glyceroltriesters and the mixtures of these compounds. Mention may in particularbe made, among the above triesters, of glycerol triesters, preferablypredominantly composed (for more than 50%, more preferably for more than80% by weight) of an unsaturated C₁₈ fatty acid, that is to say selectedfrom the group consisting of oleic acid, linoleic acid, linolenic acidand the mixtures of these acids. More preferably, whether it is ofsynthetic origin or natural origin (case, for example, of sunflower orrapeseed vegetable oils), the fatty acid used is composed for more than50% by weight, more preferably still for more than 80% by weight, ofoleic acid. Such triesters (trioleates) having a high content of oleicacid are well known; they have been described, for example, inApplication WO 02/088238 as plasticizing agents in tyre treads.

Preferably, the content of extending oil is between 2 and 50 phr, morepreferably between 3 and 40 phr and more preferably still between 5 and35 phr.

According to another preferred embodiment of the invention, thisplasticizing agent is a thermoplastic hydrocarbon resin, the Tg of whichis greater than 0° C., preferably greater than 20° C. This resin is asolid at ambient temperature (23° C.), in contrast to a liquidplasticizing compound, such as an oil.

Preferably, the thermoplastic plasticizing hydrocarbon resin exhibits atleast any one of the following characteristics:

-   -   a Tg of greater than 20° C., more preferably of greater than 30°        C.;    -   a number-average molecular weight (Mn) of between 400 and 2000        g/mol, more preferably between 500 and 1500 g/mol;    -   a polydispersity index (PI) of less than 3, more preferably of        less than 2 (as a reminder: PI=Mw/Mn with Mw the weight-average        molecular weight).

More preferably, this thermoplastic plasticizing hydrocarbon resinexhibits all of the preferred characteristics above.

The macrostructure (Mw, Mn and PI) of the hydrocarbon resin isdetermined by steric exclusion chromatography (SEC); solventtetrahydrofuran; temperature 35° C.; concentration 1 g/l; flow rate 1ml/min; solution filtered through a filter with a porosity of 0.45 μmbefore injection; Moore calibration with polystyrene standards; set of 3Waters columns in series (Styragel HR4E, HR1 and HR0.5); detection bydifferential refractometer (Waters 2410) and its associated operatingsoftware (Waters Empower).

The thermoplastic hydrocarbon resins can be aliphatic or aromatic oralso of the aliphatic/aromatic type, that is to say based on aliphaticand/or aromatic monomers. They can be natural or synthetic, based or notbased on petroleum (if such is the case, also known under the name ofpetroleum resins).

Suitable as aromatic monomers are, for example: styrene,α-methylstyrene, ortho-, meta- or para-methylstyrene, vinyltoluene,para-(tert-butyl)styrene, methoxystyrenes, chlorostyrenes,vinylmesitylene, divinylbenzene, vinylnaphthalene or any vinylaromaticmonomer resulting from a C₉ fraction (or more generally from a C₈ to C₁₀fraction). Preferably, the vinylaromatic monomer is styrene or avinylaromatic monomer resulting from a C₉ fraction (or more generallyfrom a C₈ to C₁₀ fraction). Preferably, the vinylaromatic monomer is theminor monomer, expressed as molar fraction, in the copolymer underconsideration.

According to a particularly preferred embodiment, the plasticizinghydrocarbon resin is selected from the group consisting ofcyclopentadiene (abbreviated to CPD) or dicyclopentadiene (abbreviatedto DCPD) homopolymer or copolymer resins, terpene homopolymer orcopolymer resins, terpene/phenol homopolymer or copolymer resins, C₅fraction homopolymer or copolymer resins, C₉ fraction homopolymer orcopolymer resins, α-methylstyrene homopolymer or copolymer resins andthe mixtures of these resins, which can be used alone or in combinationwith a liquid plasticizer, for example an MES or TDAE oil. The term“terpene” combines here, in a known way, α-pinene, β-pinene and limonenemonomers; use is preferably made of a limonene monomer, which compoundexists, in a known way, in the form of three possible isomers:L-limonene (laevorotatory enantiomer), D-limonene (dextrorotatoryenantiomer) or else dipentene, a racemate of the dextrorotatory andlaevorotatory enantiomers. Mention will in particular be made, among theabove plasticizing hydrocarbon resins, of α-pinene, β-pinene, dipenteneor polylimonene homo- or copolymer resins.

The preferred resins above are well known to a person skilled in the artand are commercially available, for example sold as regards:

-   -   polylimonene resins: by DRT under the name Dercolyte L120        (Mn=625 g/mol; Mw=1010 g/mol; P1=1.6; Tg=72° C.) or by Arizona        under the name Sylvagum TR7125C (Mn=630 g/mol; Mw=950 g/mol;        PI=1.5; Tg=70° C.);    -   C₅ fraction/vinylaromatic copolymer resins, in particular C₅        fraction/styrene or C₅ fraction/C₉ fraction copolymer resins: by        Neville Chemical Company under the names Super Nevtac 78, Super        Nevtac 85 and Super Nevtac 99, by Goodyear Chemicals under the        name Wingtack Extra, by Kolon under the names Hikorez T1095 and        Hikorez T1100 or by Exxon under the names Escorez 2101 and        Escorez 1273;    -   limonene/styrene copolymer resins: by DRT under the name        Dercolyte TS 105 from DRT or by Arizona Chemical Company under        the names ZT115LT and ZT5100.

Mention may also be made, as examples of other preferred resins, ofphenol-modified α-methylstyrene resins. In order to characterize thesephenol-modified resins, it should be remembered that a number referredto as “hydroxyl number” (measured according to Standard ISO 4326 andexpressed in mg KOH/g) is used in a known way. α-Methylstyrene resins,in particular phenol-modified ones, are well known to a person skilledin the art and are commercially available, for example sold by ArizonaChemical under the names Sylvares SA 100 (Mn=660 g/mol; PI=1.5; Tg=53°C.); Sylvares SA 120 (Mn=1030 g/mol; PI=1.9; Tg=64° C.); Sylvares 540(Mn=620 g/mol; PI=1.3; Tg=36° C.; hydroxyl number=56 mg KOH/g); andSilvares 600 (Mn=850 g/mol; PI=1.4; Tg=50° C.; hydroxyl number=31 mgKOH/g).

According to a specific embodiment of the invention, when it is includedin the composition, the content of plasticizing hydrocarbon resin isbetween 5 and 50 phr, preferably between 7 and 40 phr and morepreferably still between 10 and 35 phr. Preferably again, the content ofplasticizing resin is between 5 and 20 phr and more preferably between 5and 15 phr.

Of course, the compositions in accordance with the invention can be usedalone or as a blend (i.e., as a mixture) with any other rubbercomposition which can be used for the manufacture of tyres.

It is obvious that the invention relates to the rubber compositionsdescribed above both in the “raw” or non-crosslinked state (i.e., beforecuring) and in the “cured” or crosslinked, or also vulcanized, state(i.e., after crosslinking or vulcanization).

II—PREPARATION OF THE RUBBER COMPOSITIONS

The compositions are manufactured in appropriate mixers, using twosuccessive phases of preparation which are well known to a personskilled in the art: a first phase of thermomechanical working orkneading (sometimes referred to as “non-productive” phase) at hightemperature, up to a maximum temperature of between 110° C. and 190° C.,preferably between 130° C. and 180° C., followed by a second phase ofmechanical working (sometimes referred to as “productive” phase) atlower temperature, typically below 110° C., for example between 60° C.and 100° C., during which finishing phase the crosslinking orvulcanization system is incorporated; such phases have been described,for example, in Applications EP-A-0 501 227, EP-A-0 735 088, EP-A-0 810258, WO00/05300 or WO00/05301.

The first (non-productive) phase is preferably carried out in severalthermomechanical stages. During a first stage, the elastomers and thereinforcing fillers (and optionally the coupling agents and/or otheringredients) are introduced into an appropriate mixer, such as anordinary internal mixer, at a temperature between 20° C. and 100° C. andpreferably between 25° C. and 100° C. After a few minutes, preferablyfrom 0.5 to 2 min, and a rise in the temperature to 90° C. or to 100°C., the other ingredients (that is to say, those which remain, if notall were added at the start) are added all at once or in portions, withthe exception of the crosslinking system, during a mixing ranging from20 seconds to a few minutes. The total duration of the kneading, in thisnon-productive phase, is preferably between 2 and 10 minutes at atemperature of less than or equal to 180° C. and preferably of less thanor equal to 170° C.

After cooling the mixture thus obtained, the vulcanization system isthen incorporated at low temperature (typically less than 100° C.),generally in an external mixer, such as an open mill; the combinedmixture is then mixed (productive phase) for a few minutes, for examplebetween 5 and 15 min.

The final composition thus obtained is subsequently calendered, forexample in the form of a sheet or of a plaque, in particular forlaboratory characterization, or also extruded, in order to form, forexample, a rubber profiled element used for the manufacture ofsemi-finished products, in order to obtain products such as sidewalls,carcass ply, crown plies (or tyre belt), tread, bead-wire filling, treadunderlayer or other elastomer layers, preferably the tread. Theseproducts can subsequently be used for the manufacture of tyres,according to the techniques known to a person skilled in the art.

The vulcanization (or curing) is carried out in a known way at atemperature generally between 130° C. and 200° C., under pressure, for asufficient time which can vary, for example, between 5 and 90 min, as afunction in particular of the curing temperature, of the vulcanizationsystem adopted, of the kinetics of vulcanization of the compositionunder consideration or also of the size of the tyre.

The examples which follow illustrate the invention without, however,limiting it.

III—EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION III-1 Preparation ofthe Examples

In the examples which follow, the rubber compositions were produced asdescribed above.

III-2 Characterization of the Examples

In the examples, the rubber compositions are characterized, beforeand/or after curing, as indicated below.

—Mooney Viscosity or Mooney Plasticity (Before Curing):

Use is made of an oscillating consistometer as described in FrenchStandard NF T 43-005 (1991). The Mooney plasticity measurement iscarried out according to the following principle: the composition in theraw state (i.e., before curing) is moulded in a cylindrical chamberheated to 100° C. After preheating for one minute, the rotor rotateswithin the test specimen at 2 revolutions/minute and the working torquefor maintaining this movement is measured after rotating for 4 minutes.The Mooney plasticity (ML 1+4) is expressed in “Mooney unit” (MU, with 1MU=0.83 newton.metre). For greater readability, the results will beshown in base 100, the value 100 being assigned to the control. A resultof less than 100 will indicate a decrease in the value concerned and,conversely, a result of greater than 100 will indicate an increase inthe value concerned.

—Tensile Tests (after Curing):

These tests make it possible to determine the elasticity stresses andthe properties at break. Unless otherwise indicated, they are based onStandard NF ISO 37 of December 2005. The “nominal” secant moduli (orapparent stresses, in MPa, with respect to the strain, without unit) at100% elongation (“M100”) are measured in second elongation (i.e., afteran accommodation cycle). All these tensile measurements are carried outunder the standard conditions of temperature (23±2° C.) and hygrometry(50±10% relative humidity). The breaking stresses (in MPa) and theelongations at break (in %) are also measured. For greater readability,the results will be shown in base 100, the value 100 being assigned tothe control. A result of less than 100 will indicate a decrease in thevalue concerned and, conversely, a result of greater than 100 willindicate an increase in the value concerned.

—Dynamic Properties (after Curing):

The dynamic properties G* and tan(δ)max are measured on a viscosityanalyser (Metravib V A4000) according to Standard ASTM D 5992-96. Theresponse of a sample of vulcanized composition (cylindrical testspecimen with a thickness of 4 mm and a cross section of 400 mm²),subjected to a simple alternating sinusoidal shear stress, at afrequency of 10 Hz, under the standard temperature conditions accordingto Standard ASTM D 1349-99, is recorded. A peak-to-peak strain amplitudesweep is carried out from 0.1% to 50% (outward cycle) and then from 50%to 1% (return cycle). The result made use of is the loss factor tan(δ).For the return cycle, the maximum value of tan(δ) observed (tan(δ)max)is indicated. The tan(δ)max values given below are measured at 23° C.For greater readability, the results will be shown in base 100, thevalue 100 being assigned to the control. A result of less than 100 willindicate a decrease in the value concerned and, conversely, a result ofgreater than 100 will indicate an increase in the value concerned.

III-3 Examples III-3-1 Example I

The object of this example is to compare the various rubber propertiesof a control composition with compositions in accordance with theinvention, that is to say comprising a diene elastomer, a reinforcingfiller predominantly comprising silica, a crosslinking system, devoid ofor comprising less than 0.5 phr of guanidine derivative and comprising ahydroxyalkylpiperazine of formula (I) as defined above.

In particular, the compositions C-1 and C-2 were prepared and tested.The formula of the compositions C-1 and C-2 varies only in thereplacement of DPG by 1,4-bis(2-hydroxyethyl)piperazine (in an isomolaramount), as presented in Table 1. The composition C-2, in accordancewith the invention, is compared with the control composition C-1.

TABLE 1 Composition C-1 C-2 SBR (1) 100 100 Carbon black (2) 3 3 Silica(3) 110 110 Coupling agent (4) 9 9 Antioxidant (5) 2 2 Plasticizing oil(6) 12 12 Plasticizing resin (7) 46 46 Zinc oxide 1.2 1.2 DPG (8) 1.8 01,4-bis(2-Hydroxyethyl)piperazine 0 1.5 Stearic acid 2 2 Accelerator (9)2.3 2.3 Sulphur 1 1 (1) SBR (Sn star-branched) with 27% of styrene unitsand 24% of 1,2- units of the butadiene part (Tg = −48° C.) bearing asilanol functional group at the end of the elastomer chain (2) ASTMgrade N234 (Cabot) (3) Silica, Zeosil 1165 MP from Rhodia, “HDS” type(4) Coupling agent: TESPT (Si69 from Evonik-Degussa) (5)N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (Santoflex 6-PPD)from Flexsys (6) Sunflower oil comprising 85% by weight of oleic acid,Lubrirob Tod 1880 from Novance (7) C₅/C₉ Resin, Escorez 1273 from Exxon(8) Diphenylguanidine (Perkacit DPG from Flexsys) (9)N-Cyclohexyl-2-benzothiazolesulphenamide (Santocure CBS from Flexsys)

The compositions are manufactured with introduction of all of theconstituents onto an internal mixer, with the exception of thevulcanization system. The vulcanization agents (sulphur and accelerator)are introduced onto an external mixer at low temperature (theconstituent rollers of the mixer being at approximately 30° C.).

The properties measured before and after curing are given in Table 2below. For a better understanding and comparison of the tests, theresults are given in base 100, that is to say that the experimentalvalue for the control is reduced to the value 100 and then the values ofthe tests are given as a function of this base of 100 for the control.

TABLE 2 Composition No. C-1 C-2 Mooney (in base 100) 100 112 M100 (inbase 100) 100 102 Tan(δ)max (in base 100) 100 97

In comparison with the control composition C-1, a fall in the hysteresiswith regard to the tan(δ)max indicator is found for the composition C-2in accordance with the invention, which is accompanied by propertiesotherwise similar.

1.-27. (canceled)
 28. A tire comprising a rubber composition based on:at least one diene elastomer, a reinforcing filler predominantlycomposed of silica, a crosslinking system, and a hydroxyalkylpiperazineof formula (I):

in which R₁ represents a linear or branched alkyl group comprising from1 to 18 carbon atoms, R₂ represents a linear or branched alkylene groupcomprising from 1 to 18 carbon atoms, wherein R₁ and R₂ may bear one ormore additional hydroxyl groups, and wherein the rubber compositionincludes less than 0.5 phr of guanidine derivative.
 29. The tireaccording to claim 28, wherein the rubber composition includes less than0.45 phr of guanidine derivative.
 30. The tire according to claim 29,wherein the rubber composition includes less than 0.4 phr of guanidinederivative.
 31. The tire according to claim 30, wherein the rubbercomposition is devoid of guanidine derivative.
 32. The tire according toclaim 28, wherein the hydroxyalkylpiperazine is present in the rubbercomposition at a content of from 0.1 to 8 phr.
 33. The tire according toclaim 32, wherein the content of hydroxyalkylpiperazine is from morethan 0.15 phr to 7 phr.
 34. The tire according to claim 33, wherein thecontent of hydroxyalkylpiperazine is from 0.2 to 5 phr.
 35. The tireaccording to claim 34, wherein the content of hydroxyalkylpiperazine isfrom more than 0.25 phr to 4 phr.
 36. The tire according to claim 28,wherein R₁ and R₂ comprise from 2 to 12 carbon atoms.
 37. The tireaccording to claim 36, wherein R₁ and R₂ comprise from 2 to 6 carbonatoms.
 38. The tire according to claim 37, wherein R₁ and R₂ comprisefrom 2 to 4 carbon atoms.
 39. The tire according to claim 28, wherein R₁and R₂ are linear carbon-based chains.
 40. The tire according to claim28, wherein the hydroxyalkylpiperazine of formula (I) bears anadditional hydroxyl group on R₁ or R₂.
 41. The tire according to claim40, wherein the hydroxyalkylpiperazine of formula (I) bears theadditional hydroxyl group on R₁.
 42. The tire according to claim 28,wherein the silica is present in the rubber composition at a content offrom 30 to 150 phr.
 43. The tire according to claim 28, wherein thereinforcing filler comprises a minor amount of carbon black.
 44. Thetire according to claim 43, wherein the carbon black is present in therubber composition at a content of from 0.5 to 50 phr.
 45. The tireaccording to claim 28, wherein the diene elastomer is selected from thegroup consisting of: polybutadienes, synthetic polyisoprenes, naturalrubber, butadiene copolymers, isoprene copolymers, and mixtures thereof.46. The tire according to claim 28, wherein the diene elastomer ispredominantly composed of non-isoprene diene elastomer.
 47. The tireaccording to claim 46, wherein the diene elastomer is composed of 100phr of non-isoprene diene elastomer.
 48. The tire according to claim 46,wherein the non-isoprene diene elastomer is selected from the groupconsisting of: polybutadienes, butadiene copolymers, isoprenecopolymers, and mixtures thereof.
 49. The tire according to claim 28,wherein the rubber composition is additionally based on a plasticizer.50. The tire according to claim 49, wherein the plasticizer is selectedfrom the group consisting of: plasticizing resins, extending oils, andmixtures thereof.
 51. The tire according to claim 49, wherein theplasticizer is present in the rubber composition at a content of from 5to 100 phr.
 52. The tire according to claim 28, wherein the rubbercomposition comprises less than 0.5 phr zinc.
 53. The tire according toclaim 52, wherein the rubber composition comprises less than 0.3 phrzinc.
 54. The tire according to claim 53, wherein the rubber compositionis devoid of zinc.
 55. The tire according to claim 54, wherein therubber composition is additionally based on a coupling agent.
 56. Thetire according to claim 55, wherein the coupling agent is ahydroxysilane polysulphide corresponding to the general formula (III):(HO)_(a)R_((3-a))Si—R′—S_(x)R′—SiR_((3-b))(OH)_(b)  (III), wherein the Rradicals, which are identical or different, are hydrocarbon groups;wherein the R′ radicals, which are identical or different, are divalentconnecting groups; wherein a and b, which are identical or different,are equal to 1 or 2; and wherein x is a number greater than or equal to2.
 57. The tire according to claim 56, wherein the R radicals arehydrocarbon groups comprising from 1 to 15 carbon atoms.
 58. The tireaccording to claim 56, wherein the R′ radicals are divalent connectinggroups comprising from 1 to 18 carbon atoms.
 59. The tire according toclaim 56, wherein the coupling agent is a monohydroxysilane in which aand b are equal to
 1. 60. The tire according to claim 56, wherein the Rradicals are selected from the group consisting of: C₁-C₆ alkyls, C₅-C₈cycloalkyls, and a phenyl radical, and wherein the R′ radicals areselected from the group consisting of: C₁-C₁₈ alkylenes and C₆-C₁₂arylenes.
 61. The tire according claim 56, wherein the R radicals areselected from the group consisting of: C₁-C₆ alkyls and the R′ radicalsare selected from the group consisting of: C₁-C₁₀ alkylenes.
 62. Thetire according to claim 56, wherein the hydroxysilane is amonohydroxysilane polysulphide of formula (IV):

wherein the R radicals are C₁-C₃ alkyls; wherein the R′ radicals areC₁-C₄ alkylenes; and x is greater than or equal to
 2. 63. The tireaccording to claim 62, wherein the R radicals are each methyl.
 64. Thetire according to claim 62, wherein the R′ radicals are selected fromthe group consisting of: methylene, ethylene, and propylene.
 65. Thetire according to claim 56, wherein the hydroxysilane is abis(propyldimethylsilanol) polysulphide of specific formula (IVa):


66. The tire according to claim 28, wherein the rubber composition is acomposition of a tire layer selected from the group consisting of: allor part of a tread, all or part of a tire belt, and combinationsthereof.
 67. The tire according to claim 66, wherein the rubbercomposition comprises all or part of the tread.